On amplifying high-frequency signals, linearity is required between input and output signals when an envelope of the input signal is varied with time. This is because absence of linearity will cause occurrence of so-called non-linear distortion (hereafter, referred to as “distortion”). Such non-linear distortion brings about radiating unnecessary power outside a desired band for original signals and results in interference with an adjacent channel.
Actual power amplifiers however have not a little non-linearity, and it is impossible to obtain perfect linearity in them. In general, power efficiency of the power amplifier, namely, a rate of power of high-frequency signals to the power of the power source tends to be reduced when placing importance on the linearity. In contrast, the non-linearity will be increased if it is tried to increase the power efficiency. Proposal has been made about techniques of compensating the non-linear distortion caused to occur in the amplifiers, so as to alleviate a trade-off between these conflicting characteristics of the amplifiers.